I have often read reviews on Amazon that say books are life changing and I can honestly say that this is the first book I have read that I can confidently say this about.
Before reading this book I was regularly stressed, felt something was missing from my life, couldn't sleep properly and felt very down. I am now back to my old self, feeling more confident, relaxed and fearless, able to deal with whatever life throws at me. I can now live my life to the full with no regrets and a sunnier outlook.
But, I feel that it's important to say that the book itself is only as good as the 'technique' of mindfulness and that mindfulness will only work if you actually do the meditations (either using guided CDs, or on your own).
I can see why there are some negative reviews for this book as it is long and can take a while to get into, I also felt similarly when I first picked it up, but it doesn't take long to get past this. In fact, the stronger your mindfulness practice, the more useful the book becomes and then you are glad of the 500+ pages. Although some of it may seem repetitive, I really believe that it is important to see the act of mindfulness from many different perspectives and to understand how to apply it to different areas of your life.
So, in short, it's a completely life-changing concept, but will be a completely useless book unless you are willing to put in the time and commitment to making it a firm part of your life. Without this, it will just be a very long book that sits on your shelf with little meaning. I would liken it to an exercise regime, you'll only see the results if you put in the work.
It's worth noting that I have read this book as part of a MBCT course so I have been actively doing the plan outlined in the book (with daily meditations etc.) and that is why I am reviewing this particular book, but there are plenty of others on the market which are shorter and perhaps more accessible. A particularly good one is The Mindful Way Through Depression: Freeing Yourself from Chronic Unhappiness (includes Guided Meditation Practices CD)
(also relevant for those without depression), but there are many more available.